1. Field of the Invention
The present invention relates to a polymer electrolyte membrane for use in a polymer electrolyte fuel cell. More particularly, the present invention is concerned with a polymer electrolyte membrane comprising (a) a fluorinated polymer electrolyte having an ion exchange group, and (b) a basic polymer, wherein, optionally, at least a part of component (a) and at least a part of component (b) are chemically bonded to each other. The polymer electrolyte membrane of the present invention has excellent properties with respect to chemical stability, mechanical strength and heat stability, and exhibits high durability even when used at high temperatures. A polymer electrolyte fuel cell employing the polymer electrolyte membrane of the present invention is advantageous in that, even when the polymer electrolyte fuel cell is operated for a long time under high temperature/low humidity conditions (corresponding to an operating temperature around 100° C. and a humidification with 60° C. water (wherein the humidification with 60° C. water corresponds to a relative humidity (RH) of 20%)), the polymer electrolyte membrane will not suffer a breakage (such as the occurrence of a pinhole) and, therefore, a cross-leak (i.e., mixing of a fuel and an oxidant due to a breakage of a polymer electrolyte membrane) will not occur, thereby enabling the fuel cell to be operated stably for a long time under stringent conditions. The present invention is also concerned with a method for producing the above-mentioned polymer electrolyte membrane. The present invention is further concerned with a membrane/electrode assembly comprising the above-mentioned polymer electrolyte membrane which is securely sandwiched between an anode and a cathode, and concerned with a polymer electrolyte fuel cell comprising the membrane/electrode assembly.
2. Prior Art
Fuel cells generate electric energy by an electro-chemical reaction between a fuel (hydrogen source) and an oxidant (oxygen). That is, the chemical energy of the fuel is directly converted into an electric energy. As fuel sources, there can be used pure hydrogen and compounds containing hydrogen, such as petroleum, natural gases (such as methane) and methanol.
Fuel cells have the following advantageous characteristics. A fuel cell itself employs no mechanical parts and, therefore, it generates little noise. Further, in principle, a fuel cell can continue to generate electricity semipermanently by continuing to supply externally a fuel and an oxidant to the cell.
Electrolytes can be classified into a liquid electrolyte and a solid electrolyte. A fuel cell which employs a polymer electrolyte membrane as an electrolyte is called a “polymer electrolyte fuel cell”.
The polymer electrolyte fuel cells are able to operate at low temperatures, as compared to the operating temperatures of other types of fuel cells. Therefore, the polymer electrolyte fuel cells are attracting attention as promising candidates for an alternative power source of an automobile and the like, a household cogeneration system and a portable electric power generator.
A polymer electrolyte fuel cell comprises a membrane/electrode assembly comprised of a proton exchange membrane which is securely sandwiched between gas diffusion electrodes, wherein each gas diffusion electrode is a laminate comprising an electrode catalyst layer and a gas diffusion layer. The proton exchange membrane mentioned herein is a material which has strongly acidic groups (e.g., a sulfonic acid group and a carboxylic acid group) in a polymer chain thereof and allows selective permeation of protons. Examples of proton exchange membranes include perfluorinated proton exchange membranes, such as Nafion (trade name; manufactured and sold by E.I. duPont de Nemours & Company Inc., U.S.A) having high chemical stability.
For the operation of a fuel cell, a fuel (e.g., hydrogen) and an oxidant (e.g., oxygen or air) are, respectively, supplied to anodic and cathodic gas diffusion electrodes, and the two electrodes are connected to each other through an external circuit. Specifically, when hydrogen is used as a fuel, hydrogen is oxidized on the anodic catalyst to thereby generate protons, and the generated protons pass through a proton-conductive polymer in the anodic catalyst layer. Then the protons pass through the proton exchange membrane and then a proton-conductive polymer in the cathodic catalyst layer, thereby reaching the surface of the cathodic catalyst. On the other hand, electrons which are generated simultaneously with the generation of protons during the oxidation of hydrogen flow through the external circuit to thereby reach the cathodic gas diffusion electrode. On the cathodic catalyst of the cathodic gas diffusion electrode, the electrons react with both the above-mentioned protons and the oxygen in the oxidant to thereby generate water, and an electric energy is obtained by the reaction. During the operation of the fuel cell, the proton exchange membrane must function as a gas barrier. When the proton exchange membrane has high gas permeability, a leak of hydrogen from the anode side to the cathode side and a leak of oxygen from the cathode side to the anode side (namely a cross-leak) occur to cause a so-called chemical short circuiting, thus rendering it impossible to obtain a high voltage electricity.
The polymer electrolyte fuel cells are usually operated at around 80° C. so that the fuel cells can exhibit high output property. However, when a polymer electrolyte fuel cell is used in an automobile, from the viewpoint of the operation of the automobile in summer, it is desired that the fuel cell is able to operate under high temperature/low humidity conditions (corresponding to an operating temperature around 100° C. and a humidification with 60° C. water (wherein the humidification with 60° C. water corresponds to a relative humidity (RH) of 20%)). However, when a fuel cell employing a conventional perfluorinated proton exchange membrane is operated for a long time under high temperature/low humidity conditions, a problem occurs in that the proton-exchange membrane suffers the occurrence of a pinhole, thus causing a cross-leak. Therefore, the durability of the conventional proton exchange membrane is unsatisfactory.
As methods for improving the durability of the perfluorinated proton exchange membranes, there can be mentioned the following methods: a method in which a proton exchange membrane is reinforced by incorporation of polytetrafluoroethylene (PTFE) fibrils (see Unexamined Japanese Patent Application Laid-Open Specification No. Sho 53-149881 (corresponding to U.S. Pat. No. 4,218,542) and Examined Japanese Patent Application Publication No. Sho 63-61337 (corresponding to EP 94679 B); a method in which a proton exchange membrane is reinforced with a stretched porous PTFE membrane (see Examined Japanese Patent Application Publication No. Hei 5-75835 and Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. Hei 11-501964 (corresponding to U.S. Pat. Nos. 5,599,614 and 5,547,551); and a method in which a proton exchange membrane is reinforced by incorporation of inorganic particles (such as Al2O3, SiO2, TiO2 and ZrO2) (see Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 6-111827 and Hei 9-219206, and U.S. Pat. No. 5,523,181). (In the above-mentioned method in which a proton exchange membrane is reinforced using PTFE fibrils, PTFE fibrils are added to a raw material solution for producing a proton exchange membrane. In the above-mentioned method in which a proton exchange membrane is reinforced using a stretched porous PTFE membrane, a stretched porous PTFE membrane is adhered to a produced proton exchange membrane or, alternatively, a stretched porous PTFE membrane is impregnated with a raw material solution for producing a proton exchange membrane, followed by removal of the solvent from the solution to thereby produce a proton exchange membrane containing the porous PTFE membrane.) In addition, as methods for obtaining a perfluorinated proton exchange membrane having an improved heat resistance, there can be mentioned the following methods: a method in which a perfluorinated proton exchange membrane is subjected to crosslinking treatment to thereby form a crosslinkage through a strongly acidic crosslinking group (see Unexamined Japanese Patent Application Laid-Open Specification No. 2000-188013); and a method in which a sol-gel reaction is used to incorporate silica into a perfluorinated proton exchange membrane (see K. A. Mauritz, R. F. Storey and C. K. Jones, in Multiphase Polymer Materials: Blends and Ionomers, L. A. Utracki and R. A. Weiss, Editors, ACS Symposium Series No. 395, p. 401, American Chemical Society, Washington, D.C. (1989)). However, none of theses methods are able to solve the above-mentioned problem.
There is a report that a fuel cell using a proton exchange membrane comprising a material obtained by doping polybenzimidazole (having high heat resistance) with a strong acid (e.g., phosphoric acid) (hereinafter referred to as a “strong acid-doped membrane”) can be operated at a high temperature which is not less than 100° C. (see Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. Hei 11-503262 (corresponding to U.S. Pat. No. 5,525,436)). However, at an operating temperature below 100° C., liquid water is present in such fuel cell, and the doped strong acid moves from the membrane into the water, thus decreasing the output of the fuel cell. Therefore, such fuel cell is unsuitable for operation at a temperature below 100° C. For this reason, such fuel cell is difficult to use in automobiles, because a fuel cell used in an automobile is frequently switched on and off and is required to be able to operate at a temperature below 100° C.
Proton exchange membranes made of a polymer blend containing a polybenzimidazole are known. Representative examples of proton exchange membranes made of a polymer blend containing a polybenzimidazole include a proton exchange membrane produced from a polymer composition comprising a sulfonated aromatic polyether ketone and a polybenzimidazole (see Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. 2002-529546 (corresponding to U.S. Pat. No. 6,632,847)); and a proton exchange membrane obtained by a method in which a hydrocarbon polymer having an ion exchange group and a basic polymer (such as a polybenzimidazole) are blended with each other in the presence of an aprotic solvent and, then, the resultant polymer blend is cast, followed by removal of the solvent, to obtain a proton exchange membrane (see Japanese Patent Application prior-to-examination Publication (Tokuhyo) Nos. 2002-512285 (corresponding to U.S. Pat. No. 6,300,381 B1) and 2002-512291 (corresponding to U.S. Pat. No. 6,723,757)). However, these polymer electrolyte membranes produced from a polymer blend comprising a hydrocarbon polymer and a polybenzimidazole exhibit only an unsatisfactory level of chemical stability and, therefore, these polymer electrolyte membranes are unable to solve the above-mentioned problem of the occurrence of a cross-leak.
On the other hand, Comparative Example 3 of KR 2003-32321 A discloses a polymer electrolyte membrane produced from a polymer blend comprising a polybenzimidazole (PBI) and Nafion. This polymer electrolyte membrane is produced as follows. A polybenzimidazole and Nafion are individually dissolved in dimethylacetoamide to thereby obtain two solutions. The obtained two solutions are mixed together, and the resultant mixture is cast, followed by removal of the solvent, thereby obtaining a solid polymer electrolyte membrane. However, by this method, PBI cannot be uniformly microdispersed in Nafion, and the produced polymer electrolyte membrane has a non-uniform dispersion of PBI and assumes a mottled appearance. In other words, the produced polymer electrolyte membrane has many portions containing only a small amount of PBI and, hence, cannot exhibit the desired effects of PBI. Specifically, such portions of the membrane exhibit only an unsatisfactory level of chemical stability which is substantially the same as the chemical stability of Nafion as such, and such portions of the membrane are causative of the occurrence of a cross-leak. Therefore, this polymer electrolyte membrane cannot exhibit a satisfactory level of durability for use in a fuel cell which is operated under high temperature/low humidity conditions.
As described hereinabove, a polymer electrolyte membrane having excellent properties with respect to chemical stability, mechanical strength and heat stability, and exhibiting high durability even when used at high temperatures, that is, a polymer electrolyte membrane suitable for practical use, has not been obtained in the prior art. Specifically, there is no conventional polymer electrolyte membrane which can be advantageously used for producing an excellent fuel cell which is advantageous not only in that the fuel cell is free from the occurrence of a cross-leak even when the fuel cell is operated for a long time under high temperature/low humidity conditions (corresponding to an operating temperature around 100° C. and a humidification with 60° C. water (wherein the humidification with 60° C. water corresponds to a relative humidity (RH) of 20%)), but also in that the fuel cell does not suffer a lowering of the output property even when the fuel cell is frequently switched on and off. Therefore, the development of a polymer electrolyte membrane having the above-mentioned excellent properties has been desired.